Frequency converters can be used in a known manner to control the rotation of a rotating electrical machine. The control of the machine, such as a motor or a generator, is implemented by controlling the currents of the machine such that desired operation is achieved.
As the control of motor is carried out by the frequency converter that outputs voltage pulses to the motor, the parameters of the motor should be known accurately so that desired currents can be achieved. Such parameters can be, for example, inductances and resistances of the machine. The accuracy of inductances of a machine affects the calculation of fluxes and torque. Some parameters can be fed manually to the frequency converter when the drive is taken into use. Some frequency converters include an identification run, in which the frequency converter identifies automatically the parameters of the machine by feeding test pulses to the machine and calculating from the responses to those test pulses parameters of the machine.
The parameters of the machine depend on various aspects. For example, the temperature of the machine and saturation effects can change the properties of the machine such that values of identified parameters can change during the operation of the machine depending on the operating point.
Further, when the operating point of the machine is out of the typical range of operating points, the parameters can change considerably. Such operating points of the machine are not taken into account in parameter identification and as the parameters used in the control structure of the frequency converter do not match with the actual parameters, the control of the machine is not optimal.
It is known in the art of frequency converters to adapt or tune the various parameters used in the control system for making the control more accurate. One such known method is the use of signal injection. In signal injection, a high-frequency signal is injected to the output of the frequency converter. The response obtained from the injected signal is detected, and certain corrections can be made to the parameters based on the response. Another known identification technique is model reference adaptive system (MRAS) in which on the basis of a known model, a parameter of another model is adapted.
The tuning of parameters initiates the use of the actual control system, which impedes the controller operation. The controller needs to be tuned to take the injected signal into account. Further, known tuning systems obtain the correct value of a certain parameter only at the current operating point. Thus, the parameters cannot be identified in or at a desired operating point.